The state law enacted last year authorized a four-year pilot project that will expire in 2017, but so far, not a single marijuana seed has been planted. Now that the Joint Committee on Administrative Rules signed off on the guidelines, state agencies that are running the program can start posting applications for a limited number of grower and retailer permits.

Patients will be able to apply for the required identification cards starting in September.

Illinois officials haven’t publicly estimated the size of the potential market or the taxes it could generate from 21 cultivation centers selling to 60 dispensaries around the state. The Marijuana Policy Project, a Washington, D.C.-based lobbying group, estimates more than 10,000 patients could eventually sign up in Illinois.

Jonathan Caulkins of Carnegie Mellon University in Pittsburgh, a leading expert on marijuana legalization, made a rough calculation for Illinois’ annual sales of $20 million to $30 million. Growers would pay a 7 percent privilege tax on their marijuana sales and the state will collect up to $6 million in annual fees for permits.

Illinois is one of a growing number of states that has authorized the use of marijuana for medical purposes, with New York recently becoming the 23rd state earlier this month.

In early 2013 before the Illinois’ law was enacted, a poll by the Paul Simon Public Policy Institute at Southern Illinois University found that 63 percent of state residents favored legalizing medical marijuana.

Recreational marijuana is still illegal in Illinois.

The Illinois law lists dozens of medical diagnoses that can qualify a patient to use medical pot, such as cancer, glaucoma, HIV, hepatitis C, Lou Gehrig’s disease, Crohn’s disease, agitation of Alzheimer’s disease, muscular dystrophy and others.

Starting in September, patients will be able to apply for a required medical cannabis registry identification card. The Illinois Department of Public Health plans to take applications from patients with last names starting with A through L in September and October, and M through Z in November and December.

Patients will pay $100 a year to apply for a medical marijuana card. Disabled people and veterans will pay $50 annually.

Patients receiving treatment at Veterans Affairs hospitals will have a somewhat easier time getting a medical marijuana card. VA doctors, as federal employees, aren’t permitted to recommend controlled substances, so the state’s rules spell out that veterans getting VA care won’t need a doctor to sign off on their application.